In many applications it is desirable to determine the spin characteristics of a rotating object, including the object's total rate of spin (spin rate) and the axis about which the object is rotating (spin axis). For example, the spin characteristics of a golf ball in conjunction with other parameters such as ball velocity and launch angle may be used to accurately predict the trajectory followed by the golf ball after it as been struck with a golf club. Such information is useful in golf simulator applications as well as golf equipment research and development applications. The determination of spin characteristics is also useful in assessing the effects of changes in a golfer's technique or equipment on the spin characteristics of a struck golf ball as part of a training system or an equipment selection system.
Devices and methods that attempt to characterize the spin of a rotating object, such as a golf ball, are known. With some devices and methods, spin information is inferred from observations of the rebound path followed by the ball after striking a mechanical network or grid having known characteristics. In other devices and methods, permanent magnets are embedded in the ball resulting in rotating magnetic fields as the ball spins. The rotating magnetic fields may be detected by coils placed in close proximity to where the ball is struck. Other devices and methods utilize high-speed photographic or videographic techniques. Sequential images of a ball having markings on its surface are obtained. By analyzing the relative position of the markings in the sequential images either manually or with computer processing means, the spin characteristics of the ball over the time interval of the images may be obtained.
The approaches to determining the spin characteristics outlined above can present several disadvantages, particularly when utilized in golf simulation, training, equipment selection, or general research and development applications. Mechanical grids can be relatively bulky and may provide inconsistent results when different types of balls and/or ball cover compositions are used. Also, mechanical grids interrupt the flight path of the ball preventing a user from observing the actual flight path of the ball. Magnetic systems require specially manufactured balls with magnets embedded therein. The magnets can change the moment of inertia of the ball thereby reducing the suitability of such devices and methods in research and development or equipment selection applications. High-speed photographic and videographic techniques systems are typically complex and expensive. Furthermore, such systems require a high resolution image and therefore highly magnified view of the ball resulting in a spin determination over only a very limited portion of the flight path of the ball. Additionally the performance of such systems may be compromised in outdoor situations where bright sunlight can interfere with image quality.
The devices and methods outlined above also share the common disadvantage that the spin characteristics of the ball are typically measured or sampled over only a relatively short interval of time immediately after the ball is struck. However, due to drag forces, the spin rate of the ball decays over its flight path. Particularly in research and development applications where it may be desirable to characterize the aerodynamic properties of a particular ball design, the capability of monitoring the spin characteristics over a longer portion of the ball's flight path may be advantageous.